Garry’s Mod is primarily a sandbox game.

It can be enhanced with mods for additional gameplay content (I highly recommend doing so, as the base game can quickly get boring on its own).

There’s not much to say about the base gameplay, since it’s about running around maps, and spawning in as much 3D model ragdolls as your computer allows to abuse the physics engine.

The game is best played with friends (or on a multiplayer server).